Nanotechnology has generated a great impact on materials, microelectronics, computing, pharmaceuticals, medicinal, environmental, energy and the chemical industries. Nanocatalysts are an important part of nanotechnology which have found increasing commercial applications. Suitable areas include pollution and emission control technologies, such as automotive pollution abatement, catalytic removal of volatile organic compounds (VOCs) in indoor environment and low temperature air purification. Applications include, for example, using mask filters to burn CO at room temperature, chemical processing of a range of bulk and specialty chemicals, sensors to detect poisonous or flammable gases or substances in solution, and the emerging hydrogen economy for clean hydrogen production and fuel cell systems (see, Corti, C. W., et al. Applied Catalysis A: General, 2005, 291, 253).
Fuel cells offer highly efficient energy conversion with negligible pollutant emissions, which have great potential and are expected to be widely used within 10 years around the world. However, current fuel cell technology requires that the hydrogen (H2) gas used in the fuel cells, especially in Polymer Electrolyte Membrane Fuel Cells (PEMFC) have high purity to avoid poisoning the platinum (Pt) electrodes. This requirement means the CO concentration in the H2 gas should be less than 50 ppm or even 10 ppm. To achieve this goal, the H2 used for fuel cells must be pre-purified to remove the trace amount of CO and it is preferable that the process be carried out through a catalytic process.
Noble metal nanoparticles, including Au-based nanoparticles, are known to be catalytically active and potentially useful for the purification of H2 for fuel cell applications. In particular, Au-based catalysts have good activity at low temperature for selective oxidation of CO. In addition, Au has the advantage of relatively low cost compared to other noble metals, such as platinum and palladium (Pd). Despite the activity in oxidizing carbon monoxide, Au-based catalysts cannot be commercialized due to its short lifetime and the catalytic efficiency of the catalyst. The factors affecting the lifetime of the catalyst include an insufficient interaction between gold particles and the support. Recent studies have shown the catalytic activity of the metal particles is dependent on the size of the metal particles, the support used and the method of preparation (see, Haruta, M. Catalysis Today, 1997, 36, 153; Valden, M. et al. Science, 1998, 281, 1647; Grunwaldt, J. D., et al. J. of Catalysis, 1999, 181, 223;). In general, metal nanoparticles with well-controlled size/distribution on a solid support exhibit higher activity. The stability of the nanoparticles remains an area of intensive research interest. A method for the preparation of nanoparticle catalysts that have high activity, good stability and are readily produced on various solid supports is very desirable.
Traditional methods for the catalyst preparation include impregnation, incipient wetness, co-precipitation (CP) (see, Yuan, Y. et al. J. Catal. 1997, 170, 191; Gardner S. D. et al., Langmuir, 1991, 36, 153; Haruta, M. Catal. Today, 1997, 36, 153) and deposition-precipitation (see, Haruta, M. et al. J. Catal. 1993, 144, 175; Genus, J. W., In Preparation of Catalysts III (Poncelet, G. et al. Eds.), Elsevier, Amsterdam, 1983, p. 1; Zanella, R. et al. J. Phys. Chem. B 2002, 106, 7634). Impregnation and incipient wetness techniques provide poor control on Au particle size with a limited Au loading. The complete removal of chloride anion, an inhibitor to the catalyst, is also proven to be difficult (see, Ponec, V. et al., Catalysis by metals and alloys, Amsterdam 1996; Galvagno, S. et al., J. Catal., 1978, 55, 178; Cant, N. W., et al. J. Phys. Chem. 1971, 75, 178; Schwank, J. et al. J. Catal., 1980, 61, 19; Blick, K. et al. Catal. Lett., 1998, 50, 211; Sermon, P. A. et al. J. Chem. Soc. Faraday Trans. I, 1979, 40, 175). Co-precipitation (CP) and deposition-precipitation techniques, which use HAuCl4 as a precursor, can produce a highly active catalyst with high Au loading, especially when urea is used as a precipitation agent (˜8 wt %). However, the method has the limitation of consuming large quantities of water, and the reaction is carried out with heating at a high temperature (≧80° C.) from one to several hours. Since the particles are typically produced through extensive heating at an elevated temperature, solid supports materials are limited under the high temperature process. There is also a need to improve the stability of the catalysts against calcinations. Other methods including cationic adsorption or use of organogold complex are more expensive compared with the above methods. Sputtering and laser ablation methods can provide good control on Au particle size, but the apparatus can be quite expensive and the scale-up of the production is difficult (See, Fan, L. et al. In Studies In Surface Science and Catalysis 132 (Iwasawa, Y. et al. Editors) p. 769).
For the foregoing reasons, there is a need to develop a new method for the preparation of a highly active and stable metal nanoparticle catalyst with well-controlled size/distribution for CO oxidation. The method should be simple and versatile for depositing the metal nanoparticles onto various solid supports. The present invention meets these and other needs.